Air filtration media having metal nanoparticle agglomerates adhered thereto, formation thereof and use thereof

ABSTRACT

Metal nanoparticle agglomerates may convey biocidal activity to surfaces upon which they are deposited and become adhered, such as various air filtration media. Air filtration media may comprise a plurality of fibers having a plurality of metal nanoparticle agglomerates adhered thereto. The metal nanoparticle agglomerates may comprise a plurality of fused, partially fused, or unfused metal nanoparticles that are associated with one another upon a surface of the plurality of the fibers. Suitable metal nanoparticles for promoting biocidal activity against various pathogens, such as viruses and bacteria, may include copper nanoparticles and/or silver nanoparticles. Masks, inline filters, and air filtration systems may incorporate the air filtration media.

BACKGROUND

The world is facing increasing threats from antibiotic-resistant strainsof bacteria (i.e., “super bugs”) that cannot be effectively treated due,at least in part, to the overuse of antibiotics. Other types ofresistant microorganisms can present similar issues. Common influenzaand emerging viruses, such as coronaviruses, also represent asignificant health threat. Increased population densities and efficientmass transit infrastructure have further contributed significantly tolocalized and global spread of both common and emerging diseases.

Even common bacterial and viral infections may present serious healthrisks if effective infection controls are not practiced. More virulentpathogens, including the virus associated with the current coronaviruspandemic, threaten to overwhelm existing healthcare structure due toinadequate infection control.

Personal protective equipment (PPE), such as masks, may be employed aspart of protocols for establishing infection control. Masks are oftenworn by infected and healthy individuals to limit the spread of disease,particularly in crowded locales where there is an increased risk ofperson-to-person disease transmittal, particularly when effective socialdistancing is not feasible. In a healthcare setting, the use of masksand additional personal protective equipment may be even more critical.The effectiveness of masks for decreasing the incidence of infections issometimes limited, however. Masks and similar personal protectiveequipment are often characterized by their ‘N’ rating. N100-rated masks,for example, can filter out particulates down to about 0.3 microns insize. The influenza virus is smaller than this limit (˜0.14 microns),but viral transport often takes place via larger respiratory droplets(e.g., about 5 microns or larger in size) generated from coughing orsneezing, meaning that they can be effectively filtered by an N100-ratedmask, unless the respiratory droplets break up upon contact. Inaddition, respiratory droplets may evaporate quickly (i.e., withinseveral seconds) upon ejection from an individual, thereby leaving freeviruses airborne. Masks with a lower filtration efficiency, such asN95-rated masks, allow up to 5% of even these larger size particulatesto pass through. Even in the case of an N100-rated mask, sealing aroundthe mouth and nose areas is often insufficient, and coughing or sneezingcan expose gaps along the edges of the mask, thereby allowing infectedrespiratory droplets to escape or enter. As an additional concern, N100-and N99-rated masks, and even N95-rated masks to some degree, providehigh flow resistance that may make breathing difficult, especially forinfected individuals already having respiratory difficulties.Consequently, the CDC actively discourages the wearing of N95 and higherrated masks, especially by individuals not trained in their use.Although lower rated masks may be more easily worn, they may leave awearer more subject to infection. Moreover, the effectiveness of masksin promoting infection control may depend upon the proper and consistentuse of masks by individuals potentially subject to encountering areadily spread pathogen.

Another potential route of infection from masks arises from touching acontaminated mask by removing and reusing it multiple times, or simplyby adjusting a mask multiple times throughout the course of ordinarydaily use. Contamination and disease transmittal of this type may arisewith masks of any type, even N100 masks. Masks are often reused multipletimes by a user, sometimes up to a month or more in length, particularlyin view of ongoing shortages of personal protective equipment arisingdue to the current coronavirus pandemic. The high surface area of masksand the moist environment generated by passage of exhaled breaththerethrough can create a fertile breeding ground for bacteria trappedby the mask, thereby increasing the risk of spreading a disease. Trappedviruses can similarly represent a contamination hazard in used masks,particularly in combination with pathogenic or non-pathogenic bacteria.In addition, contaminated masks need to be handled carefully to preventcross-contamination when worn between one area and another. Pathogentransfer to various touch surfaces arising from adjustment ofcontaminated masks also may represent a significant concern. Used masksmay also represent a significant biohazardous waste disposal issue.Other air filtration media (e.g., HEPA filters, air conditioningfilters, HVAC or airplane/automobile cabin filters, and the like)likewise may accumulate pathogens in the course of their use, which mayextend over months or years, and lead to similar concerns as aconsequence of spreading pathogens that have accumulated on thefiltration medium.

BRIEF DESCRIPTION OF THE DRAWINGS

The following figures are included to illustrate certain aspects of thepresent disclosure, and should not be viewed as exclusive embodiments.The subject matter disclosed is capable of considerable modifications,alterations, combinations, and equivalents in form and function, as willoccur to one having ordinary skill in the art and the benefit of thisdisclosure.

FIGS. 1 and 2 show diagrams of presumed structures of metalnanoparticles having a surfactant coating thereon.

FIG. 3 shows an illustrative SEM image of substantially individualcopper nanoparticles.

FIG. 4 shows an illustrative SEM image of an agglomerate of coppernanoparticles.

FIG. 5 shows an illustrative SEM image of a copper nanoparticle networkobtained after fusion of a plurality of copper nanoparticles to eachother.

FIGS. 6A and 6B show illustrative SEM images of agglomerates of coppernanoparticles adhered to textile fibers.

FIG. 7 shows an illustrative SEM image of agglomerates of coppernanoparticles adhered to fibers of a cellulose/polyester blend (55:45),in which fiber junctions are fused together.

FIG. 8 shows an illustrative photograph of a fabric having agglomeratesof copper nanoparticles adhered thereto, as fabricated (left side ofimage) and after extended use (right side of image).

DETAILED DESCRIPTION

The present disclosure is generally directed to air filtrationtechnology, including masks, respirators, inline air filters, and likeair filtration systems, and, more specifically, to air filtration media,including systems and methods related thereto, that may limit secondaryspread of diseases once a pathogen has become trapped thereon and beenkilled and/or inactivated upon contacting an agent associated with theair filtration media.

As discussed above, masks and related air filtration media may providesome protection against the spread of viruses, bacteria and other typesof pathogens. However, the risk of secondary infection transmissionremains high, since pathogens may remain active and sequestered in orupon an air filtration medium after use. Such trapped pathogens mayfrequently spread to various touch surfaces and foster secondaryinfection transmission. Sufficiently small pathogens may pass through anair filtration medium in some instances as well.

The present disclosure provides air filtration media, such as masks,respirators and inline filters, for example, that may lessen thelikelihood of secondary transmission of pathogens trapped therein, aswell as providing enhanced protection for a wearer or user. Industrialor residential air filtration devices such as air conditioning filtersand other types of air handling systems may exhibit similar benefitsthrough application of the disclosure herein. In particular, the presentdisclosure describes various types of air filtration media that areimpregnated with a metal, such as copper and silver, having biocidalactivity toward various types of pathogens. When pathogens, such asviruses and/or bacteria, sequestered within an air filtration medium areexposed to the metal, the metal may kill or inactivate the pathogens,thereby limiting the potential for secondary spread of disease to occurand better protecting a wearer or user. In addition to mitigatingcross-contamination or transmission of pathogens and providing betterwearer or user protection, metal incorporation may limit such pathogenichazards following disposal as well, potentially decreasing wastedisposal costs in some instances. For example, masks and other personalprotective equipment incorporating a suitable metal may be disposed ofin ordinary trash rather than being sequestered as biohazardous wasterequiring more rigorous handling protocols.

Although it may be desirable to incorporate metals within air filtrationmedia, doing so may be difficult to realize in practice withconventional metal incorporation approaches. Metallic silver and copperare difficult to incorporate within an air filtration medium due to thehigh melting point of these metals. Molten copper, for instance, formsat the melting point of copper (1083° C.), a temperature which iscompletely incompatible with the filtration media commonly used in airfiltration devices and systems. The melting point of silver is likewiseproblematically high. Micron-size metal particles or flakes may beproduced and incorporated as solids to an air filtration medium, but itmay be difficult to promote sufficient adherence of the particles orflakes to an air filtration medium to afford robust performance. Inaddition, the biocidal activity of micron-size metal particles or flakesmay not be much different than the biocidal activity of a bulk metalsurface. Although both silver and copper surfaces possess some biocidalactivity against some bacteria and viruses, even against someantibiotic-resistant bacteria strains in some instances, the rate ofinactivation or killing of the pathogens may be rather slow. The slowrate of inactivation or killing of pathogens may leave open thepossibility of secondary transmission of infections from a touchsurface. Coronaviruses, for instance, may remain active for up to fivedays on surfaces such as glass, polymers, ceramics, rubber, andstainless steel, for example, and for as long as 7 days on a standardtype of surgical mask.

As a solution to the foregoing difficulties, the present disclosureprovides metal nanoparticles, particularly metal nanoparticleagglomerates, as a suitable vehicle for introducing metals intoconventional air filtration media to afford improved biocidal activityand infection control resulting therefrom. Metal nanoparticles,particularly agglomerates thereof, represent a particularly advantageousconstruct for incorporating a metal, as discussed in further detailhereinbelow. Copper nanoparticles and/or silver nanoparticles may beparticularly advantageous metal nanoparticles for conveying biocidalactivity to an air filtration medium, given the known biocidal activityof bulk copper and silver surfaces. Nanoparticle forms of these metalsmay provide an especially advantageous vehicle for incorporating copperand/or silver upon an air filtration medium, particularly air filtrationmedia comprising a plurality of fibers, due to robust surface adherence(e.g., to fibers) that may be realized when agglomerates of these metalnanoparticles are applied to the air filtration medium. Coppernanoparticles and silver nanoparticles may be used in combination withone another as well, which may afford complementary biocidal against thesame or different pathogens that may be targeted or inactivated by eachmetal individually. Zinc, nickel, titanium and other bioactive metalsmay be utilized in further combination with either of these metals aswell, including their respective oxides and/or oxides of copper and/orsilver, as well as with other additive substances that may conveypathogenic activity toward bacteria and/or viruses.

Metal nanoparticles, such as silver and copper nanoparticles, asdescribed further herein, can be readily produced as individual metalnanoparticles and/or agglomerated forms thereof that have a size rangecompatible with their ready incorporation within air filtration mediacommonly used in air filtration devices and systems. The small size ofthe metal nanoparticles and their agglomerates allows ready dispersionthroughout a desired portion of the air filtration medium to berealized, or the metal nanoparticles and/or agglomerates thereof may bedistributed in a gradient fashion upon or near one or more surfaces ofthe air filtration medium. Furthermore, due to their high surfaceenergy, metal nanoparticles may become adhered to an air filtrationmedium following deposition thereon, thereby providing a robuststructure that is capable of repeated handling during use. Adherence ofthe metal nanoparticles and agglomerates thereof may involve chemicalbond formation once the metal nanoparticles have attained a high surfaceenergy state.

Additionally, an adhesive layer to promote improved adherence of themetal nanoparticles to an air filtration medium may be furtherincorporated prior to or while applying metal nanoparticles to an airfiltration medium as well. The adhesive layer, which may be permanentlytacky, may be applied concurrently with the metal nanoparticles orseparately. Application of an adhesive layer to an air filtration mediumor portion thereof prior to deposition of metal nanoparticles thereonmay afford initial sequestration of the metal nanoparticles duringloading before more robust adherence is realized through furtherprocessing of the metal nanoparticles occurs, as discussed furtherbelow. As a further advantage, the adhesive may further promoteprolonged release of active metal species from metal nanoparticles oragglomerates thereof following their adherence to the surface of an airfiltration medium. Advantageously, metal nanoparticles or agglomeratesthereof can be applied to an air filtration medium through various spraycoating techniques, thereby allowing wide surface coverage to berealized while also affording ready control of the extent of metalloading. Alternative deposition techniques for incorporating metalnanoparticles or agglomerates thereof may also be suitable for use inthe disclosure herein.

Air filtration media of the present disclosure may advantageouslymaintain biocidal activity against various pathogens over extendedperiods of time (e.g., days to weeks). Moreover, air filtration mediahaving metal nanoparticles or agglomerates thereof adhered thereto maybe at least partially self-indicating as they lose efficacy againstpathogens during extended use. Air filtration media having active metalnanoparticle agglomerates thereon, for example, may maintain a blackcolor or similar dark hue, whereas a much lighter color may developafter the biocidal activity has diminished (see FIG. 8 ).

As used herein, the term “metal nanoparticles” refers to metal particlesthat are about 250 nm or less in size, particularly about 200 nm or lessin size or about 150 nm or less in size, without particular reference tothe shape of the metal particles. Copper nanoparticles are metalnanoparticles comprising predominantly copper, optionally with an oxidecoating. Likewise, silver nanoparticles are metal nanoparticlescomprising predominantly silver, optionally with an oxide coating. Theterm “metal nanoparticle” broadly refers herein to any metallicstructure having at least one dimension of about 250 nm or less,particularly about 200 nm or less or about 150 nm or less, and includesother structures that are not substantially spherical in nature, such asmetal platelets/disks, metal nanowires, or the like. Other metalnanostructures may be used as alternatives to or in combination withspherical or substantially spherical metal nanoparticles, oragglomerates thereof, in the disclosure herein.

The term “metal nanoparticle agglomerates” and equivalent grammaticalforms thereof refers to a grouping of metal nanoparticles having atleast one dimension ranging from about 0.1 to about 35 microns in size,particularly about 0.1 to about 15 microns in size, and moreparticularly about 0.1 to about 5 microns in size. Individual metalnanoparticles within a metal nanoparticle agglomerate may reside withinthe size ranges indicated above, and the individual metal nanoparticlesmay be associated with one another through non-covalent, covalent, ormetallic bonding interactions. The term “associated” refers to any typeof bonding force that maintains a grouping of metal nanoparticlestogether. The bonding force may be overcome to produce individual metalnanoparticles in some instances.

The terms “consolidate,” “consolidation” and other variants thereof areused interchangeably herein with the terms “fuse,” “fusion” and othervariants thereof.

The term “air filtration medium” refers to any porous construct throughwhich air, a component of air, or a gas may traverse, preferably whereinthe porous construct comprises a plurality of fibers or a fabric formedtherefrom.

Before further discussing more particular aspects of the presentdisclosure in further detail, additional brief description of metalnanoparticles and their processing conditions, particularly silver orcopper nanoparticles, will first be provided. Metal nanoparticlesexhibit a number of properties that can differ significantly from thoseof the corresponding bulk metal. One property of metal nanoparticlesthat can be of particular importance for processing is nanoparticlefusion (consolidation) that occurs at the metal nanoparticles' fusiontemperature. As used herein, the term “fusion temperature” refers to thetemperature at which a metal nanoparticle liquefies, thereby giving theappearance of melting. At or above the fusion temperature, consolidationwith other metal nanoparticles may readily take place. As used herein,the terms “fusion,” “consolidation” and other grammatical forms thereofsynonymously refer to the coalescence or partial coalescence of metalnanoparticles with one another to form a larger mass. Metalnanoparticles within a metal nanoparticle agglomerate may undergo fusionwith one another or individual metal nanoparticles may become fused aswell, thereby forming a network of at least partially fused metalnanoparticles in either case.

Advantageously and surprisingly, metal nanoparticles, such as silverand/or copper nanoparticles, can become adhered to other surfaces evenwell below the fusion temperature, thereby allowing bonding to an airfiltration medium to take place, as discussed further herein. Dependingon the density at which the metal nanoparticles are loaded onto thesurface of the air filtration medium and the temperature at which theyare processed thereon, individual metal nanoparticles may or may not befurther fused together when adhered to the air filtration medium in thedisclosure herein. Even more advantageously, metal nanoparticles mayalso be associated together with one another in the form of agglomerateswhen adhered to the air filtration medium, in which individual metalnanoparticles, which may or may not be fused together, are stillidentifiable. Metal nanoparticle agglomerates may be readily dispersedupon an air filtration medium and may likewise become adhered thereto.Advantageously, agglomeration of metal nanoparticles may facilitateretention within the air filtration media, as well as promote sustainedrelease of active metal species once adherence to an air filtrationmedium has taken place.

Upon decreasing in size, particularly below about 20 nm in equivalentspherical diameter, the temperature at which metal nanoparticles liquefydrops dramatically from that of the corresponding bulk metal. Forexample, copper nanoparticles having a size of about 20 nm or less canhave fusion temperatures of about 220° C. or below, or about 200° C. orbelow, in comparison to bulk copper's melting point of 1083° C. Silvernanoparticles may similarly display a significant deviation from themelting point of bulk silver below a nanoparticle size of about 20 nm.Thus, the consolidation of metal nanoparticles taking place at thefusion temperature can allow structures containing bulk metal to befabricated at significantly lower processing temperatures than whenworking directly with the bulk metal itself as a starting material.Since air filtration media often exhibit limited thermal tolerance,metal nanoparticles may provide a particularly effective vehicle forintroduction of metal thereto. The small particle sizes of metalnanoparticles may facilitate ready dispersion within a liquid medium forapplication to an air filtration medium, such as through a sprayingprocess. Agglomerates of the metal nanoparticles, wherein the metalnanoparticles are fused or unfused but are associated together, maylikewise be dispersible in liquid media for application to an airfiltration medium according to the disclosure herein. Metal nanoparticleagglomerates may represent a particularly effective form of metalnanoparticles for application to an air filtration medium according tothe disclosure herein due to their ready retention thereon. Oncedeposited upon a suitable air filtration medium, metal nanoparticles oragglomerates thereof may become strongly adhered thereto by increasingthe temperature to at least the fusion temperature. Adherence of metalnanoparticles or agglomerates thereof to an air filtration medium mayeven take place without increasing the temperature above the fusiontemperature to form bulk metal, sometimes even at room temperature, asdescribed further hereinbelow. Thus, metal nanoparticle agglomerateshaving at least a majority of their metal nanoparticles unfused may bepresent in the air filtration media disclosed herein. Adherence may befurther promoted with an adhesive, as discussed further herein.

A number of scalable processes for producing bulk quantities of metalnanoparticles in a targeted size range have been developed. Mosttypically, such processes for producing metal nanoparticles take placeby reducing a metal precursor in the presence of one or moresurfactants. The as-isolated metal nanoparticles may have a surfactantcoating thereon and be isolated as a plurality of nanoparticleagglomerates. The agglomerates may be broken apart, while retaining thesurfactant coating, or the agglomerates may be used directly withoutfurther processing. Particularly advantageous metal nanoparticleagglomerates may comprise metal nanoparticles ranging from about 50 nmto about 250 nm in size. In the case of adhering metal nanoparticles tothe fibers of an air filtration medium, the agglomerates may be of anadvantageous size range to facilitate dispensation via spraying and topromote retention in the air filtration medium. The metal nanoparticlesor agglomerates thereof can be isolated and purified from a reactionmixture by common isolation techniques and undergo processing into aformulation suitable for dispensation upon an air filtration medium. Thesurfactant coating upon the metal nanoparticles may be removed throughgentle heating, gas flow, and/or vacuum (any pressure below atmosphericpressure) once the metal nanoparticles have been deposited upon an airfiltration medium, thereby affording a much higher surface energy and acommensurate increase in reactivity to promote adherence to the airfiltration medium. Alternately, a surfactant coating may be lost uponextended contact with an air filtration medium without undergoingadditional heating or other processing, with adherence to the airfiltration medium occurring following surfactant loss. The metalnanoparticles may also become fused together with each other during thisprocess, or they may remain unfused. Once the surfactant coating hasbeen removed or lost, the resulting high surface energy of the metalnanoparticles may facilitate adherence of the metal nanoparticles to theair filtration medium through chemical bonding. The metal nanoparticlesmay or may not become fused to each other during the process of becomingadhered to the air filtration medium.

Metal nanoparticle agglomerates having a range of sizes, such as thosewithin a range of about 0.1 microns to about 35 microns, or about 0.1microns to about 15 microns, or about 0.1 microns to about 5 microns,may be advantageous in terms of their ability to be dispensed upon anair filtration medium, such as through aerosol formation or sprayeddroplets. In addition to their ready dispensation and beneficialretention upon an air filtration medium, additional benefits may berealized once the metal nanoparticle agglomerates have become adhered tothe surface of the air filtration medium following loss of a surfactantcoating or through bonding to an adhesive layer. In particular, metalnanoparticle agglomerates may “shed” individual metal nanoparticles orsmall clusters of metal nanoparticles that possess a high degree ofactivity against various bacteria and viruses. Once released, theindividual metal nanoparticles or small clusters of metal nanoparticlesmay migrate over the surface of the air filtration medium but withoutbeing released in vivo. By differentially releasing metal nanoparticlesfrom metal nanoparticle agglomerates having a range of sizes, atime-release profile of metal nanoparticles may be realized to affordprolonged and rapid infection control capabilities. Efficacy over theperiod of time-release may be based upon the total loading of metalnanoparticle agglomerates per unit area. Thus, activity against variouspathogens may be retained over several days, such as at least about 3days, or at least about 5 days, or at least about 7 days, or at leastabout 10 days, or at least about 14 days, or at least about 21 days, orat least about 30 days. An adhesive layer in contact with the metalnanoparticle agglomerates may further facilitate a time-release profileof metal nanoparticles for conveying biocidal activity. Suitableadhesives within an adhesive layer are not considered to be particularlylimited and are specified in more detail below.

Once a surfactant coating has been removed or lost from metalnanoparticles, an oxide coating may form thereon. Oxide-coated metalnanoparticles may also exhibit biocidal activity against bacteria andviruses, both as individual metal nanoparticles and within metalnanoparticle agglomerates. Complete conversion to metal oxide may occurin some instances. Oxidized metal nanoparticles may lead to theformation of reactive and potentially mobile salt compounds upon thesurface of an air filtration medium. Such salts may include, forexample, chlorides, bisulfites and bicarbonates. Chlorides, for example,may result from chloride ions in sweat, exhaled air or other bodilyfluids that may come into contact with the air filtration medium.Formation of such salts may be particularly prevalent upon exposure ofthe metal nanoparticles to a moist environment, as specified for abicarbonate salt in Reaction 1 below. Dry conditions, in contrast, mayfavor formation of at least a partial oxide coating upon the surface ofthe metal nanoparticles.

Cu+½O₀₂+H₂O+2CO₂→Cu(HCO₃)₂  (Reaction 1)

The salts may be surfactant-stabilized salt complexes comprising one ormore surfactants (e.g., one or more amine surfactants in the case ofcopper nanoparticles) and sufficient salt anions to achieve chargebalance. Charge balancing anions may include, for example, halogen,particularly chloride; bisulfite; bicarbonate; lactate; or the like. Thecharge balancing anions are relatively labile and may be released togenerate open coordination sites for binding DNA, proteins, or likebiomolecules, which may afford biocidal activity toward bacteria andviruses in some instances. The surfactant-stabilized salt complexes maybe relatively mobile upon the surface of an air filtration medium, evenwhen bound within an adhesive, and provide a high effective coveragethereon, even at low metal nanoparticle loadings.

Any suitable technique can be employed for forming the metalnanoparticles used in the disclosure herein. Particularly facile metalnanoparticle fabrication techniques, particularly for coppernanoparticles, are described in U.S. Pat. Nos. 7,736,414, 8,105,414,8,192,866, 8,486,305, 8,834,747, 9,005,483, 9,095,898, and 9,700,940,each of which is incorporated herein by reference in its entirety.Similar procedures may be used for synthesizing silver nanoparticles. Asdescribed therein, metal nanoparticles can be fabricated in a narrowsize range by reduction of a metal salt in a solvent in the presence ofa suitable surfactant system, which can include one or more differentsurfactants. Further description of suitable surfactant systems followsbelow. Tailoring of the surfactant system, the reaction concentration,temperature, and like factors may determine the size range of metalnanoparticles that are obtained from a metal nanoparticle synthesis.Without being bound by any theory or mechanism, it is believed that thesurfactant system can mediate the nucleation and growth of the metalnanoparticles, limit surface oxidation of the metal nanoparticles whilethe surfactant system is adhered thereto, and/or inhibit metalnanoparticles from extensively aggregating with one another prior tobeing at least partially fused together. As noted above, smallagglomerates of metal nanoparticles may be formed in many instances andused in the disclosure herein. Alternately, the metal nanoparticleagglomerates may be broken apart to form individual metal nanoparticlesor smaller metal nanoparticle agglomerates. Suitable organic solventsfor solubilizing metal salts and forming metal nanoparticles caninclude, for example, formamide, N,N-dimethylformamide, dimethylsulfoxide, dimethylpropylene urea, hexamethylphosphoramide,tetrahydrofuran, glyme, diglyme, triglyme, tetraglyme, proglyme, orpolyglyme. Reducing agents suitable for reducing metal salts andpromoting the formation of metal nanoparticles can include, for example,an alkali metal in the presence of a suitable catalyst (e.g., lithiumnaphthalide, sodium naphthalide, or potassium naphthalide) orborohydride reducing agents (e.g., sodium borohydride, lithiumborohydride, potassium borohydride, or tetraalkylammonium borohydrides).In non-limiting examples, reduction of the metal salt to form metalnanoparticles and agglomerates thereof may take place undersubstantially anhydrous conditions in a suitable organic solvent.

FIGS. 1 and 2 show diagrams of presumed structures of metalnanoparticles having a surfactant coating thereon. As shown in FIG. 1 ,metal nanoparticle 10 includes metallic core 12 and surfactant layer 14overcoating metallic core 12. Surfactant layer 14 can contain anycombination of surfactants, as described in more detail below. Metalnanoparticle 20, shown in FIG. 2 , is similar to that depicted in FIG. 1, except metallic core 12 is grown about nucleus 21, which can be ametal that is the same as or different than that of metallic core 12.Because nucleus 21 is buried deep within metallic core 12 in metalnanoparticle 20 and is very small in size, it is not believed tosignificantly affect the overall nanoparticle properties. Nucleus 21 maycomprise a salt or a metal, wherein the metal may be the same as ordifferent than metallic core 12. In some embodiments, the nanoparticlescan have an amorphous morphology. FIGS. 1 and 2 may be representative ofthe microscopic structure of individual copper or silver nanoparticlessuitable for use in the disclosure herein. FIG. 3 shows an illustrativeSEM image of substantially individual copper nanoparticles. FIG. 4 showsan illustrative SEM image of an agglomerate of copper nanoparticles,which may be used in the disclosure herein. FIG. 5 shows an illustrativeSEM image of a copper nanoparticle network obtained after fusion of aplurality of copper nanoparticles to each other. FIGS. 6A and 6B showillustrative SEM images of agglomerates of copper nanoparticles adheredto textile fibers. The agglomerates of copper nanoparticles are robustlyadhered to the textile fibers but do not undergo fusion with oneanother. FIG. 7 shows an illustrative SEM image of agglomerates ofcopper nanoparticles adhered to fibers of a cellulose/polyester blend(55:45), in which fiber junctions are fused together. The bonding totextile fibers and similar fibers may be representative of the bondingthat occurs when metal nanoparticle agglomerates contact a plurality offibers within an air filtration medium according to the presentdisclosure.

As discussed above, as-formed metal nanoparticles may have a surfactantcoating containing one or more surfactants upon their surface. Thesurfactant coating can be formed on the metal nanoparticles during theirsynthesis. Formation of a surfactant coating upon metal nanoparticlesduring their synthesis can desirably limit the metal nanoparticles fromfusing to one another prematurely, limit agglomeration of the metalnanoparticles to a desired extent or a desired agglomerate size, andpromote the formation of a population of metal nanoparticles having anarrow size distribution. At least partial loss of the surfactantcoating may occur upon heating the metal nanoparticles up to the fusiontemperature, including at least some surfactant loss well below thefusion temperature for low-boiling surfactants. Surfactant loss may befurther promoted by flowing gas and/or application of vacuum (reducedpressure), as desired, even below the fusion temperature. At least somesurfactant loss may occur at room temperature and/or atmosphericpressure in some instances, particularly over extended contact timeswith the surface of an air filtration medium. Following surfactant loss,fusion of the metal nanoparticles may take place above or below thefusion temperature. If the uncoated metal nanoparticles remain unfused,a high surface energy may be obtained within the air filtration medium,which may promote adherence thereto, such as to a plurality of fiberscomprising the air filtration medium. The metal nanoparticles may becomeadhered to the air filtration medium even below the fusion temperatureonce the surfactant coating has been removed. When heated above thefusion temperature, nanoparticle fusion may take place in combinationwith the metal nanoparticles becoming adhered to the air filtrationmedium and to each other. When copper nanoparticles and silvernanoparticles are impregnated within the air filtration medium together,fusion between the copper nanoparticles and the silver nanoparticles mayoccur as well.

Various types of metal nanoparticles may be synthesized by metalreduction in the presence of one or more suitable surfactants, such ascopper nanoparticles or silver nanoparticles. Copper and/or silver canbe particularly desirable metals for use in the embodiments of thepresent disclosure due to their ability to promote pathogen killing orinactivation when deposited upon a surface. Significantly higherbiocidal activity for both of these metals may be realized whenutilizing metal nanoparticles compared to that obtained for a bulk metalsurface. Copper may be particularly advantageous due to its low cost.Zinc and zinc oxide can similarly display biocidal activity againstbacteria, viruses and similar microorganisms and may be substituted forcopper or silver in any of the embodiments disclosed herein, or used incombination with these metals. NiO and TiO₂ may be used similarly inthis respect.

In various embodiments, the surfactant system present within the metalnanoparticles can include one or more surfactants. The differingproperties of various surfactants can be used to tailor the propertiesof the metal nanoparticles and agglomerates thereof. Factors that can betaken into account when selecting a surfactant or combination ofsurfactants for inclusion upon the metal nanoparticles can include, forexample, ease of surfactant dissipation from the metal nanoparticlesduring or prior to nanoparticle fusion, nucleation and growth rates ofthe metal nanoparticles to impact the nanoparticle size, the metalcomponent of the metal nanoparticles, the extent of metal nanoparticleagglomerate formation and size thereof, and the like. Main group metals,for example, may require different surfactants than do transition metalswhen forming metal nanoparticles.

In some embodiments, an amine surfactant or combination of aminesurfactants, particularly aliphatic amines, can be present upon themetal nanoparticles. Amine surfactants can be particularly desirable foruse in conjunction with copper nanoparticles or silver nanoparticles dueto their good affinity for these transition metals. In some embodiments,two amine surfactants can be used in combination with one another. Inother embodiments, three amine surfactants can be used in combinationwith one another. In more specific embodiments, a primary amine, asecondary amine, and a diamine chelating agent can be used incombination with one another. In still more specific embodiments, thethree amine surfactants can include a long chain primary amine, asecondary amine, and a diamine having at least one tertiary alkyl groupnitrogen substituent. This combination of surfactants may beparticularly effective for producing metal nanoparticles within adesired size range. Further disclosure regarding suitable aminesurfactants follows hereinafter.

In some embodiments, the surfactant system can include a primaryalkylamine. In some embodiments, the primary alkylamine can be a C₂-C₁₈alkylamine. In some embodiments, the primary alkylamine can be a C₇-C₁₀alkylamine. In other embodiments, a C₅-C₆ primary alkylamine can also beused. Without being bound by any theory or mechanism, the exact size ofthe primary alkylamine can be balanced between being long enough toprovide an effective inverse micelle structure during synthesis versushaving ready volatility and/or ease of handling during nanoparticleconsolidation. For example, primary alkylamines with more than 18carbons can also be suitable for use in the present embodiments, butthey can be more difficult to handle because of their waxy character.C₇-C₁₀ primary alkylamines, in particular, can represent a good balanceof desired properties for ease of use.

In some embodiments, the C₂-C₁₈ primary alkylamine can be n-hexylamine,n-heptylamine, n-octylamine, n-nonylamine, or n-decylamine, for example.While these are all straight chain primary alkylamines, branched chainprimary alkylamines can also be used in other embodiments. For example,branched chain primary alkylamines such as, for example,7-methyloctylamine, 2-methyloctylamine, or 7-methylnonylamine can beused. In some embodiments, such branched chain primary alkylamines canbe sterically hindered where they are attached to the amine nitrogenatom. Non-limiting examples of such sterically hindered primaryalkylamines can include, for example, t-octylamine,2-methylpentan-2-amine, 2-methylhexan-2-amine, 2-methylheptan-2-amine,3-ethyloctan-3-amine, 3-ethylheptan-3-amine, 3-ethylhexan-3-amine, andthe like. Additional branching can also be present. Without being boundby any theory or mechanism, it is believed that primary alkylamines canserve as ligands in the metal coordination sphere but be readilydissociable therefrom during metal nanoparticle consolidation.

In some embodiments, the surfactant system can include a secondaryamine. Secondary amines suitable for forming metal nanoparticles caninclude normal, branched, or cyclic C₄-C₁₂ alkyl groups bound to theamine nitrogen atom. In some embodiments, the branching can occur on acarbon atom bound to the amine nitrogen atom, thereby producingsignificant steric encumbrance at the nitrogen atom. Suitable secondaryamines can include, without limitation, dihexylamine, diisobutylamine,di-t-butylamine, dineopentylamine, di-t-pentylamine, dicyclopentylamine,dicyclohexylamine, and the like. Secondary amines outside the C₄-C₁₂range can also be used, but such secondary amines can have undesirablephysical properties such as low boiling points or waxy consistenciesthat can complicate their handling.

In some embodiments, the surfactant system can include a chelatingagent, particularly a diamine chelating agent. In some embodiments, oneor both of the nitrogen atoms of the diamine chelating agent can besubstituted with one or two alkyl groups. When two alkyl groups arepresent on the same nitrogen atom, they can be the same or different.Further, when both nitrogen atoms are substituted, the same or differentalkyl groups can be present. In some embodiments, the alkyl groups canbe C₁-C₆ alkyl groups. In other embodiments, the alkyl groups can beC₁-C₄ alkyl groups or C₃-C₆ alkyl groups. In some embodiments, C₃ orhigher alkyl groups can be straight or have branched chains. In someembodiments, C₃ or higher alkyl groups can be cyclic. Without beingbound by any theory or mechanism, it is believed that diamine chelatingagents can facilitate metal nanoparticle formation by promotingnanoparticle nucleation.

In some embodiments, especially suitable diamine chelating agents caninclude N,N′-dialkylethylenediamines, particularly C₁-C₄N,N′-dialkylethylenediamines. The corresponding methylenediamine,propylenediamine, butylenediamine, pentylenediamine or hexylenediaminederivatives can also be used. The alkyl groups can be the same ordifferent. C₁-C₄ alkyl groups that can be present include, for example,methyl, ethyl, propyl, and butyl groups, or branched alkyl groups suchas isopropyl, isobutyl, s-butyl, and t-butyl groups. IllustrativeN,N′-dialkylethylenediamines that can be suitable for inclusion uponmetal nanoparticles include, for example,N,N′-di-t-butylethylenediamine, N,N′-diisopropylethylenediamine, and thelike.

In some embodiments, suitable diamine chelating agents can includeN,N,N′,N′-tetraalkylethylenediamines, particularly C₁-C₄N,N,N′,N′-tetraalkylethylenediamines. The correspondingmethylenediamine, propylenediamine, butylenediamine, pentylenediamine orhexylenediamine derivatives can also be used. The alkyl groups can againbe the same or different and include those mentioned above. IllustrativeN,N,N′,N′-tetraalkylethylenediamines that can be suitable for use informing metal nanoparticles include, for example,N,N,N′,N′-tetramethylethylenediamine,N,N,N′,N′-tetraethylethylenediamine, and the like.

Surfactants other than aliphatic amines can also be present in thesurfactant system. In this regard, suitable surfactants can include, forexample, pyridines, aromatic amines, phosphines, thiols, or anycombination thereof. These surfactants can be used in combination withan aliphatic amine, including those described above, or they can be usedin a surfactant system in which an aliphatic amine is not present.Further disclosure regarding suitable pyridines, aromatic amines,phosphines, and thiols follows below.

Suitable aromatic amines can have a formula of ArNR¹R², where Ar is asubstituted or unsubstituted aryl group and R¹ and R² are the same ordifferent. R¹ and R² can be independently selected from H or an alkyl oraryl group containing from 1 to about 16 carbon atoms. Illustrativearomatic amines that can be suitable for use in forming metalnanoparticles include, for example, aniline, toluidine, anisidine,N,N-dimethylaniline, N,N-diethylaniline, and the like. Other aromaticamines that can be used in conjunction with metal nanoparticles can beenvisioned by one having ordinary skill in the art.

Suitable pyridines can include both pyridine and its derivatives.Illustrative pyridines that can be suitable for inclusion upon metalnanoparticles include, for example, pyridine, 2-methylpyridine,2,6-dimethylpyridine, collidine, pyridazine, and the like. Chelatingpyridines such as bipyridyl chelating agents may also be used. Otherpyridines that can be used in conjunction with metal nanoparticles canbe envisioned by one having ordinary skill in the art.

Suitable phosphines can have a formula of PR₃, where R is an alkyl oraryl group containing from 1 to about 16 carbon atoms. The alkyl or arylgroups attached to the phosphorus center can be the same or different.Illustrative phosphines that can be present upon metal nanoparticlesinclude, for example, trimethylphosphine, triethylphosphine,tributylphosphine, tri-t-butylphosphine, trioctylphosphine,triphenylphosphine, and the like. Phosphine oxides can also be used in alike manner. In some embodiments, surfactants that contain two or morephosphine groups configured for forming a chelate ring can also be used.Illustrative chelating phosphines can include 1,2-bisphosphines,1,3-bisphosphines, and bis-phosphines such as BINAP, for example. Otherphosphines that can be used in conjunction with metal nanoparticles canbe envisioned by one having ordinary skill in the art.

Suitable thiols can have a formula of RSH, where R is an alkyl or arylgroup having from about 4 to about 16 carbon atoms. Illustrative thiolsthat can present upon metal nanoparticles include, for example,butanethiol, 2-methyl-2-propanethiol, hexanethiol, octanethiol,benzenethiol, and the like. In some embodiments, surfactants thatcontain two or more thiol groups configured for forming a chelate ringcan also be used. Illustrative chelating thiols can include, forexample, 1,2-dithiols (e.g., 1,2-ethanethiol) and 1,3-dithiols (e.g.,1,3-propanethiol). Other thiols that can be used in conjunction withmetal nanoparticles can be envisioned by one having ordinary skill inthe art.

As mentioned above, a distinguishing feature of metal nanoparticles istheir high surface energy, particularly after removal of a surfactantcoating therefrom, which may promote adherence to an air filtrationmedium according to the disclosure herein. Robust adherence to an airfiltration medium may still be realized with the surfactant coatingintact, however, particularly when an adhesive layer is used. Metalnanoparticles, such as agglomerates of silver nanoparticles and/orcopper nanoparticles, as well as zinc nanoparticles, nickelnanoparticles, titanium nanoparticles or their oxides, optionally incombination with silver nanoparticles and/or copper nanoparticles, maybe admixed with a solvent in a spray formulation suitable for depositionupon an air filtration medium in the disclosure herein. The sprayformulations may be used to facilitate metal introduction to the airfiltration medium prior to adherence upon a surface thereof. In additionto promoting dispersion in a suitable spray formulation, one or moresurfactants associated with the metal nanoparticles as a surfactantcoating may further facilitate initial surface adhesion to the airfiltration medium prior to surfactant coating loss and formation ofuncoated metal nanoparticles having a high surface energy. That is, thesurfactant coating may initially hold metal nanoparticle agglomerates inplace until a high surface energy state has been attained to promotemore robust adherence and retention of the metal nanoparticles uponfibers of the air filtration medium.

As-produced metal nanoparticles are usually produced in the form oflarge agglomerates which need to be broken apart into smalleragglomerates and/or individual surfactant-coated metal nanoparticles inorder to promote use in various applications. Surprisingly, in thedisclosure herein, the as-produced agglomerates, such as those residingin a 0.1-35 micron size range, particularly a 1-15 micron size range ora 1-5 micron size range, can be effective for spray dispensation andretention within an air filtration medium. Agglomerates of these sizes,and even larger, may be more effectively retained within an airfiltration medium than are individual metal nanoparticles or smalleragglomerates. Such agglomerates may change their shape as they adhere toan air filtration medium, while remaining bound to each other in a“colony.” Within the agglomerates, recognizable sub-structures may bepresent prior to nanoparticle fusion such as, but not limited to, 10-50nm thick platelets having a width of about 100-250 nm, 1-5 nm thickplatelets having a width of about 30-50 nm, 100-250 nm wide spheres,metal nanowires, the like, or any combination thereof. Thesub-structures may have any shape such as square, triangular,rectangular, multi-faceted, round, and ovular, and crystalline, and/ornon-crystalline morphologies. Elongate structures, such as metalnanowires, may have an aspect ratio of at least about 10 or at leastabout 25, for example. Copper nanoparticles and/or silver nanoparticlesmay also be combined with pre-made nanowires (e.g., copper nanowires orsilver nanowires) and deposited upon an air filtration medium as well.Zinc, nickel, or titanium, particularly in the form of nanoparticles ora metal oxide form thereof, may be present in any of these embodimentsas well.

Accordingly, air filtration media of the present disclosure may comprisea plurality of fibers having a plurality of metal nanoparticleagglomerates adhered thereto, in which the metal nanoparticleagglomerates comprise a plurality of fused, partially fused, and/orunfused metal nanoparticles that are associated with one another upon asurface of the plurality of fibers. Optionally, the metal nanoparticlesmay be substantially free of a surfactant coating after becoming adheredto the plurality of fibers. In some embodiments, the metal nanoparticlesmay retain their surfactant coating when adhered to the plurality offibers and/or at least a majority of the metal nanoparticles within themetal nanoparticle agglomerates may remain unfused with one another. Inany embodiment herein, the metal nanoparticle agglomerates may beadhered to the air filtration medium via an adhesive layer, as describedfurther herein.

In the disclosure herein, the metal nanoparticles may comprise coppernanoparticles, silver nanoparticles, or any combination thereof. Withoutbeing bound by any theory or mechanism, it is believed that Cu(0) may beoxidized to Cu(I) on the air filtration medium in a slow process, withfurther oxidation to Cu(II) taking place rapidly thereafter. Whencontacting a pathogen, such as bacteria or viruses, hydroxyl radicalsand lipid radicals may form, which may disrupt the outer lipid bilayeror protein shell of a virus or bacterium. In addition, copper may bindto heteroatoms (e.g., S, N or P) within amino acids, proteins, DNAand/or RNA of viruses, bacteria and other pathogens to result ininactivation. Metal penetration within a cell membrane or protein coatmay also occur, wherein the metal may inhibit DNA/RNA replication and/orinhibit protein transport. Silver nanoparticles may promote biocidalactivity through similar mechanisms.

Combinations of copper nanoparticles and silver nanoparticles may affordparticular synergy against pathogens not remediated adequately with asingle metal alone. That is, copper nanoparticles and silvernanoparticles may convey biocidal activity against different pathogens.In addition, enhanced activity against a particular pathogen may berealized when both copper nanoparticles and silver nanoparticles arepresent, as compared to copper nanoparticles or silver nanoparticlesalone. Without being bound by theory or mechanism, two different typesof metal nanoparticles may target different biological pathways andreceptors within a pathogen, thereby affording more effective killing orinactivation than is possible with either type of metal nanoparticlealone.

Metal nanoparticles, such as silver nanoparticles and/or coppernanoparticles or agglomerates thereof, may be admixed with anaerosolizable fluid medium in spray formulations suitable for depositionupon an air filtration medium according to the disclosure herein.Suitable aerosolizable fluid media and spray formulations are describedin greater detail hereinbelow. Dip coating and similar liquid processingtechniques may also be suitable for introducing metal nanoparticles uponan air filtration medium in the disclosure herein as well.

Spray formulations comprising metal nanoparticle agglomerates, such assilver nanoparticles and/or copper nanoparticles and their agglomerates,may be prepared by dispersing as-produced or as-isolated nanoparticlesin an organic matrix containing one or more organic solvents or othermedium in which the metal nanoparticle agglomerates may be admixed as awell-dispersed solid in a fluid medium. Optionally, the fluid medium maycomprise one or more inorganic components as well, particularly water.As used herein, the term “spray formulation” refers to a fluidcomposition containing dispersed metal nanoparticles, either asindividual metal nanoparticles, agglomerated metal nanoparticles, or anycombination thereof, that is suitable for dispensation through spraying.Spray formulations refer to both pumped and forced sprays and spraysdispensed through use of an aerosol propellant. Pumped and forced spraysmay be dispensed through inert gas pressurization, and/or throughpressurization with a mechanical or pneumatic pump.

Particularly suitable organic solvents that may be present in sprayformulations suitable for dispensation by pumping or pressurizationinclude a C₁-C₁₁ alcohol, or multiple C₁-C₁₁ alcohols in anycombination. Additional alcohol-miscible organic solvents may also bepresent. Ketone and aldehyde organic solvents, also in the C₁-C₁₁ sizerange, may also be used, either alone or in combination with one or morealcohols. Ketone and aldehyde solvents are less polar than are alcoholsand may aid in promoting dispersion of metal nanoparticles and/oragglomerates thereof. Low boiling ethers such as diethyl ether, dipropylether, and diisopropyl ether, for example, may also be suitably used topromote metal nanoparticle dispersion. One or more glycol ethers (e.g.,diethylene glycol, triethylene glycol, or the like), alkanolamines(e.g., ethanolamine, triethanolamine, or the like), or any combinationthereof may also be used alone or in combination with one or morealcohols or any of the other foregoing organic solvents. Various glymesmay also be used similarly. Water-miscible organic solvents and mixturesof water and water-miscible organic solvents may be used as well, suchas water-organic solvent mixtures comprising up to about 50% water byvolume, or up to about 75% water by volume, or up to about 90% water byvolume. The organic solvent(s) may be removed either before or after thesurfactant coating is lost in the course of promoting adherence of themetal nanoparticles to the air filtration medium.

In particular examples, the spray formulations can contain one or morealcohols, which may be C₁-C₁₁, C₁-C₄, C₄-C₁₁ or C₇-C₁₁ in moreparticular embodiments. C₁-C₄ alcohols may be particularly desirable dueto their lower boiling points, which may facilitate solvent removalfollowing dispensation. In various embodiments, the alcohols can includeany of monohydric alcohols, diols, or triols. One or more glycol ethers(e.g., diethylene glycol and triethylene glycol), alkanolamines (e.g.,ethanolamine, triethanolamine, and the like), or any combination thereofmay be present in certain embodiments, which may be present alone or incombination with other alcohols. Various glymes may be present with theone or more alcohols in some embodiments.

Spray formulations suitable for dispensation by pumping or forcedpressurization with a gas may exhibit a viscosity value of about 1 cP toabout 500 cP, including about 1 cP to about 100 cP. Low viscosity valuessuch as these may facilitate dispensation through spraying promoted bymechanical pumping or forced pressurization. Metal nanoparticle loadingswithin the spray formulations to produce the foregoing viscosity valuesmay range from about 1 wt. % to about 35 wt. %, or about 5 wt. % toabout 35 wt. %, or about 10 wt. % to about 25 wt. %, or about 8 wt. % toabout 25 wt. %, or about 1 wt. % to about 8 wt. %.

Spray formulations comprising an aerosol propellant may also be suitablefor applying metal nanoparticle agglomerates to an air filtration mediumaccording to the disclosure herein. Such spray formulations maysimilarly comprise metal nanoparticles or agglomerates thereof dispersedin a fluid medium comprising at least an aerosol propellant andoptionally other solvents to promote metal nanoparticle dispersiontherein. Aerosol spray formulations may constitute a particularlydesirable form for dispensing the metal nanoparticles, since aerosolspray cans are in wide use and are easily manufactured and shipped.Aerosol propellants may afford sprayed droplets ranging from about10-150 microns in size, whereas mechanically pumped or forcedpressurization sprays may have a larger droplet size in a range of about150-400 microns.

Any conventional aerosol propellant may be utilized in a sprayformulation, provided that the metal nanoparticle agglomerates can beeffectively dispersed therein, optionally in combination with one ormore additional solvents, and ejected from a spray can. Organic and/orinorganic aerosol propellants may be used. Suitable inorganic aerosolpropellants may include, for example, nitrous oxide or carbon dioxide.Suitable organic aerosol propellants may include, for example, volatilehydrocarbons (e.g., ethane, propane, butane, or isobutane), dimethylether, ethyl methyl ether, hydrofluorocarbons, hydrofluoroolefins, orany combination thereof. Chlorofluorocarbons and similar compounds mayalso be used as an aerosol propellant, but their use is not preferreddue to their ozone-depleting properties. Nevertheless,chlorofluorocarbons may be satisfactory alternatives in situations whereother organic aerosolizable fluid media may not be effectively used.

When using an aerosol propellant to promote dispensation of metalnanoparticle agglomerates, the metal nanoparticle agglomerates may bedirectly combined therewith, or the metal nanoparticles may be dissolvedin a secondary fluid medium that is subsequently combined with theaerosol propellant in a spray can or similar container. Suitablesecondary fluid media may comprise organic solvents such as alcohols,glycols, ethers, or the like. Any of the organic solvents utilized abovein mechanically pumped or forced pressurization spray formulations maybe incorporated in spray formulations containing an aerosol propellantas a secondary fluid medium as well.

Spray formulations comprising an organic solvent may comprise a mixtureof organic solvents that evaporates in a specified period of time,typically under ambient conditions. In non-limiting examples,evaporation may take place in about 1 minute or less, or about 2 minutesor less, or about 5 minutes or less, or about 10 minutes or less, orabout 15 minutes or less, or about 30 minutes or less. To facilitateevaporation, the metal nanoparticles may be dispersed as a concentratein a higher boiling organic solvent, such as a C₁₀ alcohol, which isthen combined with a much larger quantity of low boiling organicsolvent, such as ethanol or diethyl ether, optionally in furthercombination with additional organic solvents. The high boiling organicsolvent may be sufficiently hydrophobic to facilitate dispersion of themetal nanoparticles in the less hydrophobic and lower boiling organicsolvent comprising the majority of the organic phase.

In various embodiments, the individual metal nanoparticles within themetal nanoparticle agglomerates disposed upon the air filtration mediumor present in a spray formulation may be about 20 nm or more in size,more particularly about 50 nm or more in size. In particularly suitableexamples, all or at least about 90%, at least about 95%, or at leastabout 99% of the metal nanoparticles may be about 20 nm to about 200 nmin size or about 50 nm to about 250 nm in size. Smaller coppernanoparticles (under 20 nm) may tend to undergo more extensive oxidationthan do larger metal nanoparticles, and such metal nanoparticles may bepresent to support a desired extent of oxidation. For example, smallercopper nanoparticles may tend to undergo more extensive oxidation intoCuO or Cu₂O, including partial or complete oxidation into theircompounds, than do larger copper nanoparticles having a size above 20nm. Copper nanoparticles in the foregoing size range (20 nm or above, orabout 50 nm or above) may afford a coating comprising a mixture of CuOand Cu₂O upon a metallic copper metal core, the combination of which maybe advantageous for inactivating pathogens upon an air filtration mediumaccording to the disclosure herein. Silver nanoparticles in a similarsize range may form a coating comprising silver oxide upon a metallicsilver core. When copper nanoparticles and/or silver nanoparticles areagglomerated together in the air filtration medium and adhered thereto,the oxide coating may extend over at least a portion of the surface ofthe agglomerate, leaving an exposed copper or silver metal surface belowwithin the porosity of the agglomerate. By having larger metalnanoparticles in the foregoing size range, a substantial amount ofzero-valent metal may be retained for promoting biocidal activity,whereas smaller metal nanoparticles may form too much oxide to promoteoptimal bioactivity.

Copper nanoparticles that are about 20 nm or less in size can have afusion temperature of about 220° C. or below (e.g., a fusion temperaturein the range of about 140° C. to about 220° C.) or about 200° C. orbelow, which can provide advantages for certain applications, as notedabove. Silver nanoparticles about 20 nm or less in size may similarlyexhibit a fusion temperature differing significantly from that of thecorresponding bulk metal. Larger metal nanoparticles (either copper orsilver nanoparticles), in turn, have a higher fusion temperature, whichmay rapidly increase and approach that of bulk metal as the nanoparticlesize continues to increase. Depending on the processing temperature andthe fusion temperature of the metal nanoparticles based upon their size,the metal nanoparticles may or may not be fused within the airfiltration medium when processed according to the disclosure herein. Forexample, copper and/or silver nanoparticles may remain substantiallyunfused when processed according to the disclosure herein, even if theirsurfactant coating is lost or not. Regardless of whether the metalnanoparticles are fused to each other or not after the surfactantcoating is removed, the metal nanoparticles may experience robustadherence to the air filtration medium. Surface oxidation of the metalnanoparticles may take place during this process, as discussed above.

When deposited upon an air filtration medium comprising a plurality offibers, the metal nanoparticle agglomerates may be located predominantlyupon at least one outer surface of the air filtration medium (i.e., uponan outer surface layer of the air filtration medium) or extend up to adepth of about 3-4 fiber layers in addition to the outer surface layerwhen deposited by spray coating. For example, a multi-layer airfiltration medium may have metal nanoparticle agglomerates adhered to atleast one outer layer (surface) of the air filtration medium, and one ormore inner layers may or may not contain metal nanoparticles. In anotherexample, the air filtration medium may define a removable insert withina structure not having metal nanoparticles adhered thereto. For example,the air filtration medium may comprise a removable insert for a mask(e.g., a cloth mask). The insert may be removed for washing the clothmask. The insert or any other air filtration medium disclosed herein maybe self-sterilizing due to the presence of the metal nanoparticleagglomerates. When present as an insert or similar structure, an exposedsurface of the air filtration medium having adhered metal nanoparticleagglomerates may be covered with an outer liner to preclude directexposure of the metal nanoparticle agglomerates to a wearer. Suitableliners may include porous media, such as woven or non-woven fabrics,lacking adhered metal nanoparticle agglomerates. Roll-to-roll dipcoating and gravure coating may also afford predominantly a surfacecoating of metal nanoparticle agglomerates upon an outer surface of anair filtration medium. The predominant surface coating ensures efficientuse of the metal nanoparticles for promoting biocidal activity comparedto other types of dip coating processes, wherein metal nanoparticles maybe deposited more deeply throughout predominantly all of the fiberlayers of a multi-layer fabric. Metal nanoparticle agglomerates buriedwithin deeper fiber layers may be ineffective or less effective forconveying biocidal activity since more of the metal nanoparticlesagglomerates are remote from the surface of the air filtration medium,where the loading of bacterial or viral pathogens is likely to behigher.

The loading of metal nanoparticle agglomerates upon the air filtrationmedium may include a coverage density ranging from about 0.1 mg/in² toabout 10 mg/in², or about 0.5 mg/in² to about 5 mg/in², or about 1mg/in² to about 2 mg/in² or about 0.5 mg/in² to about 3 mg/in². Thecoverage of metal nanoparticle agglomerates upon the air filtrationmedium may range from about 5% to about 95% by area, or about 50% toabout 99% by area, or about 60% to 95% by area. Even coverage densitiesas low as 3-5% by area may be effective for biocidal activity in thedisclosure herein due to the mobility of individual metal nanoparticlesor small metal nanoparticle agglomerates shed from adhered, larger metalnanoparticle agglomerates. When present at the foregoing coverages andcoverage densities upon the air filtration medium, the metalnanoparticles may effectively inactivate various pathogens, includingcertain bacteria and viruses, oftentimes more effectively than does abulk metal surface comprising the same metal. For example, coppernanoparticles adhered to an air filtration medium and retaining theirnanoparticulate form within a plurality of nanoparticle agglomerates mayinactivate/kill viruses in as little as 30 seconds. Up to 100% killrates or inactivation rates may be realized in such a short time. Bulkcopper surfaces, in contrast, may take several hours to reach the samelevel of inactivation. Bacteria may undergo similar levels ofinactivation or killing in various instances.

In addition to metal nanoparticle agglomerates or alternativenanostructures, other additives may be incorporated upon the airfiltration medium and/or within spray formulations suitable forproducing an air filtration medium according to the disclosure herein.Suitable additives may include, but are not limited to, those capable ofproducing reactive oxygen species (ROS), which may cause lipid, protein,or DNA damage in microorganisms, eventually leading to cell membranedamage and cell death. These additives may complement or enhance thebiocidal activity conveyed by copper nanoparticles, silvernanoparticles, or alternative metal nanoparticles having biocidalactivity, such as those comprising zinc, nickel, titanium, and/or theiroxide forms.

NiO may be included as an additive upon the filtration medium or withinspray formulations suitable for producing the filtration medium. NiO isvery efficient in producing ROS when present in small concentrations.NiO may be effective when included at, for example, about 0.5% to about10% of the load of copper nanoparticles and/or silver nanoparticles in aspray formulation (e.g., 0.5 mg to 100 mg NiO) as sub-micron particlesseparate distinct from the copper nanoparticles and/or silvernanoparticles. At these loadings, NiO is very effective against certainbacteria, which may broaden the biocidal effectiveness of copper orsilver. Bismuth, zinc, and tin oxides may be similarly effective atloadings of about 0.5% to about 10% of the mass of copper nanoparticlesand/or silver nanoparticles.

TiO₂ may be included as an additive upon the filtration medium or withinspray formulations suitable for producing the filtration medium. TiO₂may catalyze the formation of hydroxyl radicals upon UV irradiation(e.g., in sunlight) when the filtration medium is taken outdoors.Moisture from a wearer's breath or atmospheric moisture may supply thesource of water for producing the hydroxyl radicals by photooxidation.TiO₂ may be present at about 1% to about 25% of the load of coppernanoparticles and/or silver nanoparticles in a spray formulation or uponan air filtration medium. The TiO₂ may likewise be present in the formof nanoparticles and/or micron-size particles (e.g., about 100 nm toabout 5 microns).

Copper nanoparticles and/or silver nanoparticles, ZnO, NiO and/or TiO₂may also be used in combination with one another as well. Theseadditives may be sprayed upon the filtration medium at the same time ascopper nanoparticles and/or silver nanoparticles (from the same sprayformulation or different spray formulations), or may be sprayed beforeor after the copper nanoparticles and/or silver nanoparticles.

After depositing the spray formulation upon the filtration medium,removal of the solvent and optionally surfactants may take place.Although solvents and surfactants may be removed under ambientconditions (room temperature and atmospheric pressure), application ofat least one of heating, gas flow, and/or vacuum (reduced pressure) mayaccelerate removal of the solvent and surfactants from the airfiltration medium, thereby leading to the metal nanoparticleagglomerates becoming adhered to the air filtration medium. Heating maytake place at any temperature up to or beyond the fusion temperature ofthe metal nanoparticles, provided that the heating temperature is not sohigh that the air filtration medium itself experiences thermal damage.Thus, the metal nanoparticles may be fused or unfused when adhered tothe air filtration medium. Moreover, the heating temperature need notnecessarily exceed the normal boiling point or reduced pressure boilingpoint of the surfactants and solvents in order to promote their removal.Gentle heating well below the boiling point of the surfactant andsolvent may be sufficient to promote their removal in many instances. Innon-limiting embodiments, the heating may be conducted under flowingnitrogen, air or other inert gas or under vacuum to promote removal. Forexample, heating the air filtration medium at a temperature of about 35°C. to about 65° C. in flowing nitrogen or air may be sufficient toremove the solvent and surfactant, thereby leaving unfused metalnanoparticles distributed throughout the air filtration medium as aplurality of metal nanoparticle agglomerates. Additional heating may beconducted thereafter, if desired, to promote metal nanoparticle fusion.In either case, after the surfactants are removed from the nanoparticlesurface, robust adherence to the air filtration medium may be realized.When heating under higher temperatures, use of an inert atmosphere, suchas nitrogen, may be desirable to limit degradation of the air filtrationmedium and to control the amount of surface oxidation taking place uponthe metal nanoparticles once the surfactant coating has been removed.

Once the surfactant coating has been removed from the metalnanoparticles, particularly copper nanoparticles and/or silvernanoparticles, the metal nanoparticles and/or agglomerates thereof mayundergo at least partial oxidation. As indicated above, in the case ofcopper nanoparticles, the size of the copper nanoparticles and theagglomerates thereof may be selected such that at least some coppermetal remains following oxidation, since a mixture of copper metal(metallic copper) and oxidized copper may be beneficial for promotingpathogen inhibition or killing. Silver nanoparticles may similarlyexperience different amounts of surface oxidation depending upon thesize of the silver nanoparticles and how they are processed. Innon-limiting embodiments, following surfactant removal, coppernanoparticles may form a reaction product within an air filtrationmedium comprising about 25% to about 99% metallic copper by weight orabout 45% to about 90% metallic copper by weight, about 0.5% to about60% Cu₂O by weight, and about 0.1% to about 80% CuO by weight or about0.1% to about 20% CuO by weight. In more particular embodiments, theamount of metallic copper may be about 45% to about 90% by weight, orabout 50% to about 70% by weight, and the amount of Cu₂O may be about10% by weight or less, such as about 0.1% to about 10% by weight or lessor about 5% to about 10% by weight or less, and the amount of CuO may beabout 1% by weight or less, such as about 0.1% to about 1% by weight orabout 0.5% to about 1% by weight. The Cu₂O and CuO may form a coating(shell) upon the metal nanoparticles or agglomerates thereof that isabout 10 nm or greater in thickness, or 100 nm or greater in thickness,such as about 100 nm to about 3 microns thick in many instances.

Silver nanoparticles adhered to the air filtration medium may similarlycomprise about 25% to about 99% metallic silver by weight and thebalance being Ag₂O. The Ag₂O may similarly be present in a coating(shell) having a thickness of about 10 nm or greater, such as about 100nm to about 3 microns thick.

In addition to metal nanoparticles and other additives, the sprayformulations and air filtration media disclosed herein may furthercomprise an adhesive that is suitable for promoting nanoparticleadherence to fibers within the air filtration media. That is, the airfiltration media disclosed herein may also have an adhesive layerthereon that may further enhance adherence to fibers comprising the airfiltration media. When an adhesive layer is present, the metalnanoparticle agglomerates may become adhered to the air filtrationmedium, even without removal of the surfactant coating taking place. Theadhesive layer may be applied with the metal nanoparticle agglomerates(i.e., in a suitable spray formulation or dip coating formulation) oralready be present upon the air filtration medium before the metalnanoparticle agglomerates are applied thereto. Both contact andnon-contact adhesives may be employed for this purpose. Suitableadhesives will be familiar to one having ordinary skill in the art andinclude conventional epoxy adhesives, nitrile rubber adhesives, acrylicadhesives, styrene-acrylic adhesives, cyanoacrylate adhesives,solvent-based adhesives, aqueous emulsions, and the like. The adhesivemay be present at a loading of about 0.1 mg/in² to about 0.5 mg/in² uponthe air filtration medium. Suitable loadings of the adhesive in thespray formulations or similar formulations may range from about 0.35 gadhesive/100 g spray formulation to about 2.75 g adhesive/100 g sprayformulation. Coverage of the adhesive layer upon the air filtrationmedium may range from about 50% to about 100% by area, or about 60% toabout 90% by area, or about 75% to about 95% by area, or about 90% toabout 99% by area. A layer thickness of the adhesive layer upon the airfiltration medium may be about 300 nm or less, such as about 1 nm toabout 2 nm, or about 2 nm to about 5 nm, or about 5 nm to about 10 nm,or about 10 nm to about 50 nm, or about 10 nm to about 300 nm. Inaddition to promoting surface adherence, the adhesive may slow down theproduction of oxidized metal species, thereby affording furthertailoring of the time-release profile of individual or smallagglomerates of metal nanoparticles or various oxidized forms thereof.

When applying an adhesive layer to the surface of an air filtrationmedium, the adhesive may be present in a spray formulation or dipcoating formulation applied to the air filtration medium, or an adhesiveformulation and a spray formulation or dip coating formulationcomprising metal nanoparticles may be applied separately. The adhesiveformulation may be applied upon the air filtration medium first,followed by the spray formulation or dip coating formulation comprisingmetal nanoparticles, or the adhesive formulation and the metalnanoparticles may be applied concurrently. As a still further option, anadhesive may be applied to the air filtration medium after depositingmetal nanoparticle agglomerates thereon.

Air filtration media on which the metal nanoparticle agglomerates may beapplied and adhered according to the disclosure herein are notconsidered to be particularly limited. Any conventional air filtrationmedium may be treated with metal nanoparticles, particularly coppernanoparticles and/or silver nanoparticles, according to the disclosureherein. Illustrative air filtration media may comprise a plurality offibers, which may be natural or synthetic fibers such as cellulosic,cotton, glass or polymer fibers, for example. Suitable polymer fibersmay include, but are not limited to, polyester, polypropylene,polystyrene, or any combination thereof. Fiber combinations may define afiber blend having metal nanoparticle agglomerates adhered thereto.Suitable air filtration media that may be impregnated with metalnanoparticles according to the disclosure herein include, for example,media rated N95, N99, or N100. Lower grade masks, such as surgicalmasks, cloth masks and dust masks, for example, also may have metalnanoparticle agglomerates adhered thereto according to the disclosureherein. The air filtration medium may be in a form of a fabric, tape,sheet, film, or any combination thereof. In addition, the air filtrationmedium may be in the form of a removable insert, which may be insertedwithin a mask not otherwise having antiseptic activity conveyed theretoby the presence of metal nanoparticle agglomerates.

Fabrics comprising a plurality of fibers may be woven or non-woven innature, single-layered or multi-layered, and/or pleated or non-pleated.The term “fabric” refers to both regular and irregular arrangements ofindividual fibers, which may or may not be further bonded together.Woven fabrics may be formed by arranging individual fibers together in asubstantially regular pattern without adhering the fibers together.Non-woven fabrics, in contrast, may be formed by arranging individualfibers together and bonding the fibers together through chemical,mechanical, heat, or solvent treatment. Pleating within an airfiltration medium may increase the amount of contact surface area forcontact with air or gas. Any of the foregoing types of fabrics utilizedin the disclosure herein. In particular examples, a suitable fabric maybe multi-layered and metal nanoparticle agglomerates may be distributedin a concentration gradient among multiple layers of the fabric.

The structure and/or type of air filtration medium undergoingimpregnation with metal nanoparticle agglomerates according to thedisclosure herein is likewise not considered to be particularly limited.Air filtration media located within masks (e.g. dust masks and surgicalmasks), respirators, inline filters for air handlers, car or airplanecabin filters, medical filters, biomedical research filters, HEPAfilters, or the like may be impregnated with metal nanoparticleagglomerates, such as copper nanoparticles and/or silver nanoparticles,according to the disclosure herein to convey biocidal activity thereto.Air filtration systems may comprise at least one filter comprising theair filtration media disclosed herein. The air filtration medium maycomprise a fabric, which may be woven, non-woven, and/or melt-blown withvarying porosity in the disclosure herein. The air filtration medium maybe present in the form of an insert in some instances.

Loading of metal nanoparticle agglomerates may be substantially uniformthroughout an air filtration medium, or the metal nanoparticleagglomerates may be localized at or an outer surface of the airfiltration medium (e.g., within the top 3-4 fabric layers of amulti-layer fabric), optionally with an outer liner present thereon. Fora dust mask or similar personal protective equipment, a loading of metalnanoparticle agglomerates within the air filtration medium may be about100 mg to about 2 g, for example. Larger commercial air filters mayincorporate a commensurately larger amount of copper nanoparticlesand/or silver nanoparticles in agglomerated form. Suitable loads ofmetal nanoparticle agglomerates upon the air filtration medium may rangefrom about 0.5 wt. % to about 20 wt. %, or about 1 wt. % to about 15 wt.%, each based upon a total weight of the air filtration medium.Considerations for determining a suitable metal (e.g., copper or silver)loading upon the air filtration medium include, for example, the lengthor number of times the air filtration medium is intended to be used andthe overall flux of air passing therethrough over the period of intendeduse.

Masks, for example, may comprise a dome shape suitable for snug fittingaround the face, mouth and nose of a wearer. The mask may have anoverall area of about 28-30 square inches, with the loading of copper orsilver ranging from about 0.5 mg/in² to about 70 mg/in², or about 6.5mg/in² to about 25 mg/in², or about 3.5 mg/in² to about 15 mg/in², orabout 0.5 mg/in² to about 10 mg/in², or about 1 mg/in² to about 2.5mg/in², or about 3.5 mg/in² to about 6 mg/in². Suitable masks may besingle-layered or comprise multiple layers of air filtration media thatare bonded together at the edges. An adhesive may be included to bondthe layers together and/or to facilitate adherence of the metalnanoparticle agglomerates to the air filtration medium. Coppernanoparticles and/or silver nanoparticles may be sprayed onto an innerlayer of the mask, and then an outer liner may be placed upon the innerlayer of the filtration medium, such that the outer liner intercedesbetween the metal nanoparticles and a wearer of the mask. Thisarrangement may limit the accidental inhalation of metal nanoparticlesand/or additives from the mask. The air filtration medium may also bepresent as an insert that is placed between layers of a mask, such as acloth mask. Alternately, the air filtration medium of a mask may besingle-layered or multi-layered and have at least one outer surface ofthe air filtration medium with metal nanoparticle agglomerates adheredthereto. The outer surface facing away from a wearer and/or the outersurface facing a wearer of the mask may have metal nanoparticleagglomerates adhered thereto. Such preparations may be readilyincorporated into current manufacturing processes for masks of varioustypes.

Inline filters may similarly comprise a multi-layer structure containingmetal nanoparticle agglomerates adhered to fibers. The multi-layerstructure may comprise a pleated or non-pleated multi-layer fabric, forexample. The inline filter may include the metal nanoparticleagglomerates upon an outer surface of the air filtration medium, or themetal nanoparticle agglomerates may be adhered to one or more innerlayers of a multi-layer fabric, which are then covered with an outerliner through which an airflow may pass. Again, the outer liner may aidin trapping metal nanoparticles incidentally released from the fiberssuch that they do not travel further in an air filtration systemcontaining the inline filter, such as ductwork of an air conditioningsystem or the lines of a gas handling system.

In view of the disclosure above, methods for forming an air filtrationmedium having metal nanoparticle agglomerates adhered thereto maycomprise: providing an air filtration medium comprising a plurality offibers; applying a plurality of metal nanoparticle agglomerates to theair filtration medium, in which the plurality of metal nanoparticleagglomerates comprises a plurality of metal nanoparticles having asurfactant coating thereon when applied to the plurality of fibers; andadhering the plurality of metal nanoparticle agglomerates to theplurality of fibers. Optionally, the methods may comprise removing thesurfactant coating from the plurality of metal nanoparticles such thatthe plurality of metal nanoparticle agglomerates become adhered to theplurality of fibers. In other non-limiting examples, the metalnanoparticle agglomerates may become adhered to the plurality of fibersby an adhesive layer, in which case the surfactant layer may remainintact. When adhered to the plurality of fibers, the plurality of metalnanoparticles may comprise a plurality of fused, partially fused, and/orunfused metal nanoparticles that are associated with one another upon asurface of the plurality of fibers. In non-limiting examples, applyingthe plurality of metal nanoparticle agglomerates to the air filtrationmedium may comprise spraying a metal nanoparticle formulation onto theair filtration medium. Removing the surfactant coating, when performed,may comprise applying heat, gas flow, vacuum, or any combination thereofto the air filtration medium after applying the plurality of metalnanoparticle agglomerates thereto.

Treatment of an airflow, including exhaled air from a wearer of a maskor similar piece of personal protective equipment, may compriseproviding an air filtration medium of the present disclosure; passing anair flow through the air filtration medium, in which the air flow has apathogenic load before passing through the air filtration medium; anddecreasing the pathogenic load, inactivating or killing one or morepathogens, or any combination thereof upon passing the air flow throughthe air filtration medium. The one or more pathogens may compriseCovid-19 in particular examples.

Embodiments disclosed herein include:

A. Air filtration media. The air filtration media comprise: a pluralityof fibers having a plurality of copper nanoparticles or silvernanoparticles adhered thereto, the copper nanoparticles or silvernanoparticles being fused, partially fused, or unfused upon a surface ofthe fibers, and the copper nanoparticles or silver nanoparticles beingsubstantially free of a surfactant coating when adhered to the fibers.

A1. Dust masks comprising the air filtration media of A.

A2. Inline air filters comprising the air filtration media of A.

B. Methods for loading copper or silver upon a filtration medium. Themethods comprise: providing a filtration medium comprising a pluralityof fibers; applying a plurality of copper nanoparticles or silvernanoparticles to the filtration medium upon a surface of the fibers, thecopper nanoparticles or silver nanoparticles comprising a surfactantcoating thereon; and removing the surfactant coating from the coppernanoparticles or silver nanoparticles, such that the coppernanoparticles or silver nanoparticles become adhered to the plurality offibers.

Each of embodiments A, A1, A2 and B may have one or more of thefollowing additional elements in any combination:

Element 1: wherein the copper nanoparticles or silver nanoparticles aresubstantially free of an amine coating.

Element 2: wherein the fibers comprise cellulosic fibers, cotton fibers,polymer fibers, or any combination thereof.

Element 3: wherein the copper nanoparticles or silver nanoparticlesrange in size from about 20 nm to about 150 nm.

Element 4: wherein the silver nanoparticles are aggregated as aplurality of copper or silver nanoparticle agglomerates having a sizeranging from about 1 micron to about 5 microns.

Element 5: wherein the silver nanoparticles comprise metallic silver andsilver oxide when substantially free of the surfactant coating, or thecopper nanoparticles comprise metallic copper and one or more copperoxides when substantially free of the surfactant coating.

Element 6: wherein the filtration medium further comprises silvernanoparticles or copper nanoparticles adhered to the plurality of fibers

Element 7: wherein the copper nanoparticles comprise about 25% to about99% by weight metallic copper, 0.5% to about 60% Cu₂O by weight, andabout 0.1% to about 20% CuO by weight.

Element 8: wherein the copper nanoparticles comprise about 45% to about90% by weight metallic copper, 0.5% to about 60% Cu₂O by weight, andabout 0.1% to about 20% CuO by weight.

Element 9: wherein a loading of silver nanoparticles upon the pluralityof fibers ranges from about 0.5 wt. % to about 20 wt. %.

Element 10: wherein applying the plurality of copper nanoparticles orsilver nanoparticles to the filtration medium comprises spraying acopper nanoparticle or silver nanoparticle formulation onto thefiltration medium.

Element 11: wherein removing the surfactant coating comprises applyingheat, vacuum, or any combination thereof to the filtration medium afterapplying the plurality of copper nanoparticles or silver nanoparticlesthereto.

Element 12: wherein the surfactant coating comprises one or more amines.

Element 13: wherein the copper nanoparticles or silver nanoparticles areaggregated as a plurality of copper or silver nanoparticle agglomerateshaving a size ranging from about 1 micron to about 5 microns whenapplied to the filtration medium.

By way of non-limiting example, exemplary combinations applicable to A,A1 and A2 include: 1 and 2; 1 and 3; 1 and 4; 1, 3 and 4; 1 and 5; 1, 5and 6; 1, 5, 6, and 7 or 8; 1 and 9; 2 and 3; 2 and 4; 2-4; 2 and 5; 2,5 and 6; 2, 5, 6, and 7 or 8; 2 and 9; 3 and 4; 3 and 5; 3, 5 and 6; 3,5, 6, and 7 or 8; 5 and 6; 5-7; 5, 6 and 8; 5 and 9; 5, 6 and 9; 6, 7and 9; 6, 8 and 9; and 8 and 9. By way of further non-limiting example,exemplary combinations applicable to B include, but are not limited to,2 and 3; 2 and 5; 2, 5 and 6; 2, 5, 6, and 7 or 8; 3 and 5; 3, 5 and 6;3, 5, 6, and 7 or 8; 5, 6 and 7; 5, 6 and 8; 5 and 9; 5, 6, and 7 or 8;5, 6 and 9; 6, 7 or 8, and 9, any of which may be in further combinationwith one or more of 10, 11, 12 or 13. Additional exemplary combinationsapplicable to B include, but are not limited to, 2 and 10; 2 and 11; 2and 12; 2 and 13; 3 and 10; 3 and 11; 3 and 12; 3 and 13; 5 and 10; 5and 11; 5 and 12; 5 and 13; 5, 6 and 10; 5, 6 and 11; 5, 6 and 12; 5, 6and 13; 5, 6, 7 or 8, and 10; 5, 6, 7 or 8, and 11; 5, 6, 7 or 8, and12; and 5, 6, 7 or 8, and 13.

Additional embodiments disclosed herein include:

A′. Air filtration media. The air filtration media comprise: a pluralityof fibers having a plurality of metal nanoparticle agglomerates adheredthereto, the metal nanoparticle agglomerates comprising a plurality offused, partially fused, and/or unfused metal nanoparticles that areassociated with one another upon a surface of the plurality of fibers.

A1′. Masks comprising the air filtration medium of A′.

A2′. Inline air filters comprising the air filtration medium of A′

A3′. Air filtration systems comprising at least one filter comprisingthe air filtration medium of A′.

B′. Methods for forming an air filtration medium. The methods comprise:providing an air filtration medium comprising a plurality of fibers;applying a plurality of metal nanoparticle agglomerates to the airfiltration medium, the plurality of metal nanoparticle agglomeratescomprising a plurality of metal nanoparticles having a surfactantcoating thereon when applied to the plurality of fibers; and adheringthe plurality of metal nanoparticle agglomerates to the plurality offibers; wherein the plurality of metal nanoparticle agglomeratescomprise a plurality of fused, partially fused, and/or unfused metalnanoparticles that are associated with one another upon a surface of theplurality of fibers.

C′: Methods for treating an air flow. The methods comprise: providing anair filtration medium comprising a plurality of fibers having aplurality of metal nanoparticle agglomerates adhered thereto, the metalnanoparticle agglomerates comprising a plurality of fused, partiallyfused, and/or unfused metal nanoparticles that are associated with oneanother upon a surface of the plurality of fibers; passing an air flowthrough the air filtration medium, the air flow having a pathogenic loadbefore passing through the air filtration medium; and decreasing thepathogenic load, inactivating or killing one or more pathogens, or anycombination thereof upon passing the air flow through the air filtrationmedium.

Each of embodiments A′, A1′, A2′, A3′, B′ and C′ may have one or more ofthe following additional elements in any combination:

Element 1′: wherein the air filtration medium is multi-layered, and atleast one outer layer of the air filtration medium has metalnanoparticle agglomerates adhered thereto.

Element 2′: wherein the air filtration medium defines a removable insertfor the mask.

Element 3′: wherein the metal nanoparticles within the metalnanoparticle agglomerates comprise copper nanoparticles, silvernanoparticles, or any combination thereof.

Element 4′: wherein the metal nanoparticle agglomerates further compriseNiO, ZnO, TiO₂ or any combination thereof.

Element 5′: wherein the plurality of fibers comprise cellulosic fibers,cotton fibers, polymer fibers, glass fibers, or any combination thereof.

Element 6′: wherein at least a majority of the metal nanoparticleswithin the metal nanoparticle agglomerates range from about 20 nm toabout 250 nm in size or from about 50 nm to about 250 nm in size.

Element 7′: wherein the metal nanoparticle agglomerates range from about0.1 micron to about 35 microns in size, or wherein the metalnanoparticle agglomerates range from about 0.1 micron to about 35microns in size when applied to the air filtration medium.

Element 8′: wherein the metal nanoparticles are silver nanoparticlescomprising metallic silver and a silver oxide coating.

Element 9′: wherein the metal nanoparticles are copper nanoparticlescomprising metallic copper and a coating comprising Cu₂O, CuO or anycombination thereof.

Element 10′: wherein the copper nanoparticles comprise about 25% toabout 99% metallic copper by weight, about 0.5% to about 60% Cu₂O byweight, and about 0.1% to about 20% CuO by weight or about 0.1% to about80% CuO by weight.

Element 11′: wherein the copper nanoparticles comprise about 45% toabout 90% metallic copper by weight, about 0.5% to about 60% Cu₂O byweight, and about 0.1% to about 20% CuO by weight or about 0.1% to about80% CuO by weight.

Element 12′: wherein a loading of metal nanoparticles upon the pluralityof fibers ranges from about 0.5 wt. % to about 20 wt. % based on a totalweight of the air filtration medium.

Element 13′: wherein the plurality of fibers collectively define a wovenfabric, a non-woven fabric, or any combination thereof.

Element 14′: wherein the woven or non-woven fabric is multi-layered, andthe metal nanoparticle agglomerates are distributed in a concentrationgradient among multiple layers of the woven or non-woven fabric.

Element 15′: wherein the metal nanoparticles are substantially free of asurfactant coating after becoming adhered to the plurality of fibers.

Element 16′: wherein at least a majority of the metal nanoparticles inthe metal nanoparticle agglomerates are unfused with one another.

Element 17′: wherein the air filtration medium is self-sterilizing.

Element 18′: wherein the plurality of metal nanoparticle agglomeratesare adhered to the plurality of fibers via an adhesive layer.

Element 19′: wherein applying the plurality of metal nanoparticleagglomerates to the air filtration medium comprises spraying a metalnanoparticle formulation onto the air filtration medium.

Element 20′: wherein the method further comprises removing thesurfactant coating from the plurality of metal nanoparticles, such thatthe plurality of metal nanoparticle agglomerates become adhered to theplurality of fibers.

Element 21′: wherein removing the surfactant coating comprises applyingheat, gas flow, vacuum, or any combination thereof to the air filtrationmedium after applying the plurality of metal nanoparticle agglomeratesthereto.

Element 22′: wherein the surfactant coating comprises one or moreamines.

Element 23′: wherein at least a majority of the metal nanoparticles inthe metal nanoparticle agglomerates are unfused with one another.

By way of non-limiting example, exemplary combinations applicable to A′,A1′, A2′, A3′ and C include, but are not limited to, 1′ and 3′; 1′, 3′and 4′; 1′ and 6′; 1′ and 7′; 1′, 9′, and 10′ or 11′; 1′ and 14′; 1′ and16′; 1′ and 17′; 1′ and 18′; 1′ and 22′; 1′ and 23′; 3′ and 6′; 3′ and7′; 3′, 9′, and 10′ or 11′; 3′ and 14′; 3′ and 16′; 3′ and 17′; 3′ and18′; 3′ and 22′; 3′ and 23′; 6′ and 7′; 6′, 9′, and 10′ or 11′; 6′ and14′; 6′ and 16′; 6′ and 17′; 6′ and 18′; 6′ and 22′; 6′ and 23′; 7′, 9′,and 10′ or 11′; 7′ and 14′; 7′ and 16′; 7′ and 17′; 7′ and 18′; 7′ and22′; 7′ and 23′; 9′, and 10′ or 11′; 9′ and 14′; 9′ and 16′; 9′ and 17′;9′ and 18′; 9′ and 22′; 9′ and 23′; 14′ and 16′; 14′ and 17′; 14′ and18′; 14′ and 22′; 14′ and 23′; 6′ and 17′; 16′ and 18′; 16′ and 22′; 16′and 23′; 18′ and 22′; 18′ and 23′; and 22′ and 23′. Any of the foregoingare applicable to B′, optionally in further combination with one or moreof 19′-23′.

To facilitate a better understanding of the present disclosure, thefollowing examples of preferred or representative embodiments are given.In no way should the following examples be read to limit, or to define,the scope of the invention.

Examples

Agglomerates of copper nanoparticles in the 50-250 nm size range with amonolayer of amine surfactants on their surfaces and having anagglomerate size of 1-35 microns were adhered to a 55/45cellulose/polyester fabric blend with an average fiber diameter of about10 microns using an epoxy adhesive. This may be done via spray coating asuitable ink or dye formulation onto the fibers, or dip coating orgravure coating via a commercial process. The adhesive layer was about20-50 nm thick, and the metal nanoparticle agglomerates were partiallyembedded in the adhesive layer with a substantial portion still exposed.The areal coverage of the agglomerates upon the fiber surfaces was about20-50%. The copper loading upon the fabric ranged from about 1.2 mg/in²to about 2.7 mg/in². Depending on size, some of the agglomerates mayhave the surfactant layer partially removed, thereby resulting inpartial oxidation and an overall mixture of copper metal, Cu₂O and CuOspecies on the fiber surface. The copper metal to oxide ratio may residein the 1-10% range. Over time, oxidation and dissolution progressivelyresult in fading of the initial dark brown-red color to more lightyellow-green. FIG. 8 shows an illustrative photograph of a fabric havingagglomerates of copper nanoparticles adhered thereto, as fabricated(left side of image) and after extended use (right side of image). Thenanoparticle-loaded fabric was then subjected to various stability andtoxicological tests specified below.

Agglomerates of copper nanoparticles in the 20-150 nm size range with apartially removed monolayer of amine surfactants on their surfaces andhaving an agglomerate size of 5-15 microns were adhered to a 30/70cellulose/polyester fabric blend with an average fiber diameter of about10 microns using an epoxy adhesive. This may be done via spray coating asuitable ink or dye formulation onto the fibers, or dip coating orgravure coating via a commercial process. The adhesive layer was about50-100 nm thick and the metal nanoparticle agglomerates were partiallyembedded in the adhesive layer with a substantial portion still exposed.The areal coverage of the agglomerates upon the fiber surfaces is about30-70%. The copper loading upon the fabric ranged from about 2.3 mg/in²to about 4.5 mg/in². Depending on size, some of the agglomerates may befully or partially oxidized, thereby resulting in an overall mixture ofcopper metal, Cu₂O and CuO species on the fiber surface. The coppermetal to oxide ratio may reside in the 5-25% range.

Agglomerates of copper nanoparticles in the 50-250 nm size range with amonolayer of amine surfactants on their surfaces and having anagglomerate size of 1-35 microns were adhered to a 55/45cellulose/polyester fabric blend with an average fiber diameter of about10 microns using a styrene acrylic acid block copolymer adhesive. Thismay be done via spray coating a suitable ink or dye formulation onto thefibers, or dip coating or gravure coating via a commercial process. Theadhesive layer was about 100-250 nm thick, and the metal nanoparticleagglomerates were partially embedded in the adhesive layer with asubstantial portion still exposed. The areal coverage of theagglomerates upon the fiber surfaces is about 10-35%. The copper loadingupon the fabric ranged from about 1.7 mg/in² to about 3.5 mg/in².Depending on size, some of the agglomerates may be fully or partiallyoxidized, thereby resulting in an overall mixture of copper metal, Cu₂Oand CuO species on the fiber fabric surface. The copper metal to oxideratio may reside in the 5-15% range.

Agglomerates of copper nanoparticles in the 50-200 nm size range with amonolayer of amine surfactants on their surfaces and having anagglomerate size of 1-35 microns were adhered to a 100% polypropylenefabric (melt-blown) with an average fiber diameter of about 10 micronsusing an epoxy adhesive. This may be done via spray coating a suitableink or dye formulation onto the fibers, or dip coating or gravurecoating via a commercial process. The adhesive layer was about 35-150 nmthick, and the metal nanoparticle agglomerates were partially embeddedin the adhesive layer with a substantial portion still exposed. Theareal coverage of the agglomerates on the fiber surfaces is about 5-30%.The copper loading upon the fabric ranged from about 0.7 mg/in² to about1.6 mg/in². Depending on size, some of the agglomerates may be fully orpartially oxidized, thereby resulting in an overall mixture of coppermetal, Cu₂O and CuO species on the fiber surface. The copper metal tooxide ratio may reside in the 1-5% range.

Agglomerates of copper nanoparticles in the 35-200 nm size range with amonolayer of amine surfactants on their surfaces and having anagglomerate size of 3-25 microns were adhered to a 100% cotton fabricwith an average fiber diameter of about 10 microns using a styreneacrylic acid block copolymer adhesive. This may be done via spraycoating a suitable ink or dye formulation onto the fibers, or dipcoating or gravure coating via a commercial process. The adhesive layerwas about 50-150 nm thick, and the metal nanoparticle agglomerates werepartially embedded in the adhesive layer with a substantial portionstill exposed. The areal coverage of the agglomerates upon the fibersurfaces was about 40-75%. The copper loading upon the fabric rangedfrom about 2.7 mg/in² to about 4.5 mg/in². Depending on size, some ofthe agglomerates may be fully or partially oxidized, thereby resultingin an overall mixture of copper metal, Cu₂O and CuO species on the fibersurface. The copper metal to oxide ratio may be in the 3-25% range.

Agglomerates of copper nanoparticles in the 20-150 nm size range with amonolayer of amine surfactants on their surfaces were mixed with 35-100nm size CuO particles at a weight ratio of 25-50%. The agglomerates(5-15 microns in size) were adhered to a 30/70 cellulose/polyesterfabric blend with an average fiber diameter of about 10 microns using anepoxy adhesive. This may be done via spray coating a suitable ink or dyeformulation onto the fibers, or dip coating or gravure coating via acommercial process. The adhesive layer was about 50-200 nm thick, andthe agglomerates were partially embedded in the adhesive layer with asubstantial portion still exposed. The areal coverage of theagglomerates upon the fiber surfaces was about 30-70%. The copper/copperoxide loading upon the fabric ranged from about 1.3 mg/in² to about 2.4mg/in².

Agglomerates of copper nanoparticles in the 20-250 nm size range with amonolayer of amine surfactants upon their surface were mixed with 35-100nm size CuO particles at a weight ratio of 25-50%, along with NiO or ZnOat a 2-10% weight ratio. The agglomerates (5-15 microns in size) wereadhered to a 30/70 cellulose/polyester fabric blend with an averagefiber diameter of about 10 micron using an epoxy adhesive. This may bedone via spray coating of a suitable ink or dye formulation onto thefibers, or dip coating or gravure coating via a commercial process. Theadhesive layer was about 50-150 nm thick, and the agglomerates werepartially embedded in the adhesive layer with a substantial portionstill exposed. The areal coverage of the agglomerates upon the fibersurfaces was about 20-50%. The overall metal/metal oxide loading uponthe fabric ranged from about 1.3 mg/in² to about 2.4 mg/in².

Stability testing. A 6″×6″ sheet of fabric was tumbled in water for 8hours. Only 1.4% of the available copper by weight (0.54 mg) wasreleased into the water.

Shedding was also determined by exposing the fabric to simulatedbreathing conditions (8.4 and 40.8 m/min face velocity gas flow) andanalyzing a filter trap for liberated copper by SEM or EDS. The sheddingtests did not reveal detectable liberation of copper from the fabric.

VOCs. No volatile organic compounds (VOCs) from a battery of 70 standardVOCs were detected as being released from the fabric when tested understandard conditions.

Direct exposure to cell growth media. A piece of fabric was first soakedin supplemented cell growth media for up to an hour and then removed.Thereafter, Vero cells or Calu-3 lung epithelial cells were immersed inthe cell growth media and incubated overnight in a CO₂ incubator. Cellviability was determined by assessing ATP production using aluminescence assy. The luminescence assay did not reveal a substantialchange in cell viability.

Efficacy. Efficacy of the fabric against a panel of bacterial and viralpathogens was tested. The panel included gram-positive, gram-negative,and antibiotic-resistant bacteria, bacteriophages as representatives ofnon-enveloped viruses, enveloped viruses such as H1N1 flu, H3N2 flu, andSARS-CoV-2, and non-enveloped viruses such as feline calicivirus. In allcases, >99% kill rates were observed within 30 seconds, and fullefficacy was maintained over 15 days of repeated daily exposure. Theefficacy was >99.9% over a standard EPA exposure time of 2 hours againstStaphylococcus aureus (ATCC 6538), Enterobacter aerogenes (ATCC 13048),Pseudomonas aeruginosa (ATCC 15442), Methicillin ResistantStaphylococcus aureus MRSA (ATCC 33592), and Escherichia coli O157:H7(ATCC 35150). The fabric maintained substantially 100% of the originalefficacy against repeated viral inoculations (27M PFUs; H1N1, H3N2 andfeline calicivirus) or bacterial loads introduced to the fabric over thecourse of 30 days. The fabric maintained >99.9% efficacy againstStaphylococcus aureus and Klebsiella aerogenes after months of dailyhigh-touch use and moisture exposure with visible wear. An inactivationrate of substantially 100% was realized against human wound pathogenssuch as Acinetobacter baumannii, Klebsiella pneumonia, Pseudomonasaeruginosa, Enterococcus faecalis, Methicillin-resistant Staphylococcusaureus (MRSA), and Staphylococcus epidermidis over 24 hours.

Unless otherwise indicated, all numbers expressing quantities ofingredients, properties such as molecular weight, reaction conditions,and so forth used in the present specification and associated claims areto be understood as being modified in all instances by the term “about.”Accordingly, unless indicated to the contrary, the numerical parametersset forth in the following specification and attached claims areapproximations that may vary depending upon the desired propertiessought to be obtained by the embodiments of the present invention. Atthe very least, and not as an attempt to limit the application of thedoctrine of equivalents to the scope of the claim, each numericalparameter should at least be construed in light of the number ofreported significant digits and by applying ordinary roundingtechniques.

One or more illustrative embodiments incorporating the features of thepresent disclosure are presented herein. Not all features of a physicalimplementation are described or shown in this application for the sakeof clarity. It is understood that in the development of a physicalembodiment incorporating the present disclosure, numerousimplementation-specific decisions must be made to achieve thedeveloper's goals, such as compliance with system-related,business-related, government-related and other constraints, which varyby implementation and from time to time. While a developer's effortsmight be time-consuming, such efforts would be, nevertheless, a routineundertaking for those of ordinary skill in the art and having benefit ofthis disclosure.

Therefore, the present disclosure is well adapted to attain the ends andadvantages mentioned as well as those that are inherent therein. Theparticular embodiments disclosed above are illustrative only, as thepresent disclosure may be modified and practiced in different butequivalent manners apparent to those skilled in the art having thebenefit of the teachings herein. Furthermore, no limitations areintended to the details of construction or design herein shown, otherthan as described in the claims below. It is therefore evident that theparticular illustrative embodiments disclosed above may be altered,combined, or modified and all such variations are considered within thescope and spirit of the present invention. The disclosure hereinsuitably may be practiced in the absence of any element that is notspecifically disclosed herein and/or any optional element disclosedherein. While compositions and methods are described in terms of“comprising,” “containing,” or “including” various components or steps,the compositions and methods can also “consist essentially of” or“consist of” the various components and steps. All numbers and rangesdisclosed above may vary by some amount. Whenever a numerical range witha lower limit and an upper limit is disclosed, any number and anyincluded range falling within the range is specifically disclosed. Inparticular, every range of values (of the form, “from about a to aboutb,” or, equivalently, “from approximately a to b,” or, equivalently,“from approximately a-b”) disclosed herein is to be understood to setforth every number and range encompassed within the broader range ofvalues. Also, the terms in the claims have their plain, ordinary meaningunless otherwise explicitly and clearly defined by the patentee.Moreover, the indefinite articles “a” or “an,” as used in the claims,are defined herein to mean one or more than one of the element that itintroduces.

1. An air filtration medium comprising: a plurality of fibers having aplurality of metal nanoparticle agglomerates adhered thereto, the metalnanoparticle agglomerates comprising a plurality of fused, partiallyfused, and/or unfused metal nanoparticles that are associated with oneanother upon a surface of the plurality of fibers.
 2. The air filtrationmedium of claim 1, wherein the metal nanoparticles within the metalnanoparticle agglomerates comprise copper nanoparticles, silvernanoparticles, or any combination thereof.
 3. (canceled)
 4. (canceled)5. The air filtration medium of claim 1, wherein at least a majority ofthe metal nanoparticles within the metal nanoparticle agglomerates rangefrom about 50 nm to about 250 nm in size.
 6. The air filtration mediumof claim 1, wherein the metal nanoparticle agglomerates range from about0.1 micron to about 35 microns in size.
 7. (canceled)
 8. The airfiltration medium of claim 1, wherein the metal nanoparticles are coppernanoparticles comprising metallic copper and a coating comprising Cu₂O,CuO or any combination thereof.
 9. The air filtration medium of claim 8,wherein the copper nanoparticles comprise about 25% to about 99%metallic copper by weight, about 0.5% to about 60% Cu₂O by weight, andabout 0.1% to about 80% CuO by weight.
 10. (canceled)
 11. (canceled) 12.The air filtration medium of claim 1, wherein the plurality of fiberscollectively define a woven fabric, a non-woven fabric, or anycombination thereof.
 13. The air filtration medium of claim 12, whereinthe woven or non-woven fabric is multi-layered, and the metalnanoparticle agglomerates are distributed in a concentration gradientamong multiple layers of the woven or non-woven fabric.
 14. The airfiltration medium of claim 1, wherein the metal nanoparticles aresubstantially free of a surfactant coating after becoming adhered to theplurality of fibers.
 15. The air filtration medium of claim 1, whereinat least a majority of the metal nanoparticles in the metal nanoparticleagglomerates are unfused with one another.
 16. (canceled)
 17. The airfiltration medium of claim 1, wherein the plurality of metalnanoparticle agglomerates are adhered to the plurality of fibers via anadhesive layer.
 18. A mask, removable insert for a mask, or inlinefilter comprising the air filtration medium of claim
 1. 19. (canceled)20. (canceled)
 21. (canceled)
 22. (canceled)
 23. A method comprising:providing an air filtration medium comprising a plurality of fibers;applying a plurality of metal nanoparticle agglomerates to the airfiltration medium, the plurality of metal nanoparticle agglomeratescomprising a plurality of metal nanoparticles having a surfactantcoating thereon when applied to the plurality of fibers; and adheringthe plurality of metal nanoparticle agglomerates to the plurality offibers; wherein the plurality of metal nanoparticle agglomeratescomprise a plurality of fused, partially fused, and/or unfused metalnanoparticles that are associated with one another upon a surface of theplurality of fibers.
 24. The method of claim 23, wherein the metalnanoparticles within the metal nanoparticle agglomerates comprise coppernanoparticles, silver nanoparticles, or any combination thereof. 25.(canceled)
 26. The method of claim 23, wherein applying the plurality ofmetal nanoparticle agglomerates to the air filtration medium comprisesspraying a metal nanoparticle formulation onto the air filtrationmedium.
 27. The method of claim 23, further comprising: removing thesurfactant coating from the plurality of metal nanoparticles, such thatthe plurality of metal nanoparticle agglomerates become adhered to theplurality of fibers; wherein removing the surfactant coating comprisesapplying heat, gas flow, vacuum, or any combination thereof to the airfiltration medium after applying the plurality of metal nanoparticleagglomerates thereto.
 28. (canceled)
 29. (canceled)
 30. The method ofclaim 23, wherein at least a majority of the metal nanoparticles withinthe metal nanoparticle agglomerates range from about 50 nm to about 250nm in size.
 31. (canceled)
 32. (canceled)
 33. (canceled)
 34. The methodof claim 23, wherein at least a majority of the metal nanoparticles inthe metal nanoparticle agglomerates are unfused with one another. 35.The method of claim 23, wherein the plurality of metal nanoparticleagglomerates are adhered to the plurality of fibers via an adhesivelayer.
 36. A method comprising: providing an air filtration mediumcomprising a plurality of fibers having a plurality of metalnanoparticle agglomerates adhered thereto, the metal nanoparticleagglomerates comprising a plurality of fused, partially fused, and/orunfused metal nanoparticles that are associated with one another upon asurface of the plurality of fibers; passing an air flow through the airfiltration medium, the air flow having a pathogenic load before passingthrough the air filtration medium; and decreasing the pathogenic load,inactivating or killing one or more pathogens, or any combinationthereof upon passing the air flow through the air filtration medium. 37.The method of claim 36, wherein the metal nanoparticles within the metalnanoparticle agglomerates comprise copper nanoparticles, silvernanoparticles, or any combination thereof.
 38. (canceled)
 39. The methodof claim 36, wherein at least a majority of the metal nanoparticleswithin the metal nanoparticle agglomerates range from about 50 nm toabout 250 nm in size.
 40. (canceled)
 41. (canceled)
 42. (canceled) 43.(canceled)
 44. The method of claim 36, wherein at least a majority ofthe metal nanoparticles in the metal nanoparticle agglomerates areunfused with one another.
 45. The method of claim 36, wherein theplurality of metal nanoparticle agglomerates are adhered to theplurality of fibers via an adhesive layer.